It is a factory subwoofer that came with the vehicle, OR, did he add it as a Mopar accessory?

If it was added as a Mopar accessory, then you will need to get the wiring harness from him.

If it is a factory install, then you will NOT have a wiring harness and installation is going to be more difficult as you cannot buy a subwoofer harness from Jeep. If your friend is re-doing his whole system, would he be willing to give you the factory connector for the sub (he would have to cut this off the factory harness).

Wiring involves running a power and ground wire to the engine bay, and running audio signal wires either up to the dash or back to the amp.